Diesel engines are operated with highly superstoichiometric air/fuel mixtures. Their exhaust gas contains a correspondingly high proportion of oxygen of approximately 3 to 15% by volume. As pollutants, diesel engines emit nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC) and soot. In comparison with gasoline engines, their emissions of nitrogen oxides are low, but in order to meet legal emissions limit values, it is necessary to further reduce even these low concentrations by means of exhaust-gas aftertreatment.
On account of the high oxygen content of the diesel exhaust gas, the nitrogen oxides contained therein cannot, as in the case of stoichiometrically operated spark-ignition engines, be continuously reduced by means of three-way catalytic converters with the simultaneous oxidation of hydrocarbons and carbon monoxide to form nitrogen. So-called nitrogen oxide storage catalytic converters have therefore been developed in order to remove the nitrogen oxides from the lean exhaust gas of said engines, which nitrogen oxide storage catalytic converters store the nitrogen oxides contained in the lean exhaust gas in the form of nitrates.
The mode of operation of nitrogen oxide storage catalytic converters is described in detail in the SAE document SAE 950809. Nitrogen oxide storage catalytic converters are accordingly composed of a catalyst material which is usually applied in the form of a coating on an inert honeycomb body composed of ceramic or metal, a so-called carrier body. The catalyst material contains the nitrogen oxide storage material and a catalytically active component. The nitrogen oxide storage material is in turn composed of the actual nitrogen oxide storage component which is deposited on a support material in highly disperse form. As storage components, use is made predominantly of the basic oxides of the alkali metals, the earth alkali metals and the rare earth metals, but in particular barium oxide, which react with the nitrogen dioxide to form the corresponding nitrates.
As catalytically active components, use is usually made of the noble metals of the platinum group, which are deposited generally together with the storage component on the support material. As support material, use is made predominantly of active, high-surface-area aluminium oxide. The catalytically active components can however also be applied to a separate support material such as for example active aluminium oxide.
The task of the catalytically active components is to convert carbon monoxide and hydrocarbons in the lean exhaust gas into carbon dioxide and water. Said catalytically active components also catalyze the oxidation of nitrogen monoxide to form nitrogen dioxide, which reacts with the basic storage materials to form nitrates (storage phase or lean operation). Nitrogen monoxide does not form nitrates with the storage materials. Said nitrogen monoxide is contained in the exhaust gas of diesel engines in proportions of 65 to 95% depending on the operating conditions of the engine.
With increasing accumulation of the nitrogen oxides into the storage material, the remaining storage capacity of the storage material decreases, and said storage material must therefore be regenerated from time to time. For this purpose, the engine is operated for a short time with rich air/fuel mixtures (so-called regeneration phase or rich operation). Under the reducing conditions in the rich exhaust gas, the nitrates are broken down to form nitrogen oxides (NOx) and, using carbon monoxide, hydrogen and hydrocarbons as reducing agents, are reduced to form nitrogen with the formation of water and carbon dioxide.
During operation of the nitrogen oxide storage catalytic converter, the storage phase and regeneration phase alternate regularly. On account of the relatively low nitrogen oxide concentration in the exhaust gas, the storage phase usually lasts between 1 and 10 minutes, while the regeneration phase ends even in less than 20 seconds. In order to determine the optimum switching time from the storage phase to the regeneration phase, it is for example possible for a nitrogen oxide sensor to be arranged downstream of the storage catalytic converter. If the nitrogen oxide concentration in the exhaust gas measured by said sensor rises above a previously defined threshold value, then the regeneration of the catalytic converter is initiated. The threshold value is generally selected from the interval between 30 and 100, preferably between 30 and 60, vol.-ppm.
Modern nitrogen oxide storage catalytic converters have a working range of between approximately 150 and 500° C. Below 150° C., effective storage of the nitrogen oxides no longer takes place on account of the decreasing speed of the oxidation from nitrogen monoxide to nitrogen dioxide with decreasing temperature and on account of the slowed solid state reactions in the catalytic converter. Above 500° C., the nitrogen oxides which are stored as nitrates are no longer stable and are released into the exhaust gas as nitrogen oxides. The optimum operating range of a nitrogen oxide storage catalytic converter is approximately between 300 and 400° C. This applies both to the storage of the nitrogen oxides in the lean exhaust gas and also to the regeneration of the storage catalytic converter in the rich exhaust gas.
One peculiarity of the diesel exhaust gas is its low temperature. In part-load operation of the engine, the exhaust-gas temperature is usually between 120 and 250° C. Only at full load can it occasionally rise up to 500° C. It is therefore often the case that the exhaust gas of the diesel engine is between 120 and 250° C. during longer operating phases in city driving. In this case, one notices that an upcoming regeneration of the storage catalytic converter does not take place completely, but rather that a certain proportion of the nitrogen oxides remains on the storage catalytic converter. The storage capacity of said storage catalytic converter for nitrogen oxides is thus reduced.
The reduced storage capacity naturally leads to shortened storage phases. After the regeneration and the switch to lean operation, the threshold value of the nitrogen oxide concentration downstream of the catalytic converter for the initiation of the next regeneration is exceeded more quickly than in the case of a fully-regenerated storage catalytic converter.
If the exhaust-gas temperature is increased before the start of the regeneration in order to regenerate the storage catalytic converter as completely as possible at the increased exhaust-gas temperature, then during the temperature increase, an early emission of nitrogen oxides occurs which cannot be converted. In addition, during the storage phase, undesired nitrogen oxide emissions are observed during brief temperature increases as a result of thermal desorption of nitrogen oxides. Said undesired emissions reduce the attainable nitrogen oxide conversion.